a) Field of the Invention
The present invention relates to a zoom lens system which is to be used with video cameras or the similar instruments employing solid-state image pickup or equivalent devices.
b) Description of the Prior Art
In general zoom lens systems which have wide field angles have offaxial rays which are incident at a high point on front lens components of these zoom lens systems, and the front lens components have large diameters, whereby lens units disposed on the object side in the zoom lens systems have negative refractive powers in most cases. Further, each of conventional zoom lens systems comprise four or more lens units in order to obtain a high vari-focal ratio.
Examples of these conventional zoom lens systems are the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 59-208,519, Kokai Publication No. Sho 62-153,913 and so on.
Further, a general method for focusing the zoom lens systems of this type is to move a front lens units disposed therein. In the zoom lens systems disclosed by Japanese Patents Kokai Publication No. Sho 52-84,754, etc., the front lens unit is divided into units are divided to subunits and only certain of the subunits are moved for focusing the zoom lens systems. In this way the weight of the lens units which are movable for focusing the zoom lens systems are reduced.
The conventional zoom lens systems mentioned above hardly allows any reduction in the diameters of the front lens components to be used therein and cannot provide high quality images over the entire zooming range thereof due to an improper selections of loci for moving the lens units for zooming and an inadequate refractive power distribution in the zoom lens systems.
Furthermore, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 52-84,754 is not sufficiently compact and the imaging performance thereof for photographing objects located at short distances is due to an improper focusing method or an inadequate composition of the focusing lens unit and improper shapes of the focusing lens elements.